The present invention relates to an active-matrix liquid crystal display device.
In liquid crystal display devices, displaying is performed by varying the optical properties of the liquid crystal layer disposed between the substrates by changing the alignment direction of the liquid crystal molecules in the layer by applying an electric field thereto.
The conventional active-matrix liquid crystal displays have predominantly been of the twisted nematic (TN) mode in which the direction of the electric field applied to the liquid crystal molecules is set to be substantially vertical to the substrate plane, and display is performed by making use of optical rotatory power of the liquid crystals.
On the other hand, a system which makes use of the birefringence effect of the liquid crystals by setting the direction of the electric field applied to the liquid crystals to be substantially parallel to the substrate plane by using interdigital electrodes (in-plane switching mode) has been proposed in, for instance, JP-B 63-21907 and WO 91/10936 (JP-T 5-505247). This in-plane switching mode has the advantages of wide viewing angle and low load capacity in comparison with the conventional TN mode, and is a promising technique for the development of active-matrix liquid crystal displays.
In this in-plane switching mode, however, since it utilizes the birefringence effect of the liquid crystals for making display, it is necessary to set the gap between the substrates (liquid crystal layer thickness) to be about 4 μm, which is notably smaller than that of the TN mode (about 10 μm), for obtaining a display performance equal to the TN mode. Generally, reduction of the layer thickness brings into relief the influence of display irregularity due to nonuniformity of the gap between the substrates, giving rise to such problems as deterioration of displayed image quality and reduction of yield resulting in lowered mass productivity.
The gap between the substrates is controlled to a specified value by dispersing the uniformly sized spherical polymer beads as spacer of the opposing substrates between which the liquid crystal layer is disposed.
In the active-matrix liquid crystal display devices, a level difference of up to about 1 μm may be produced on the substrate surface at the active element forming section, and a certain degree of nonuniformity of the inter-substrate gap is inevitably produced at the pixel region, too, due to a delicate relation between said level difference and dispersion of said spacer beads.
In the in-plane switching mode, the same degree of gap irregularity represents a far greater rate of gap variation than in the TN mode because of smaller inter-substrate gap, so that the techniques for lessen-ing or eliminating display irregularity due to non-uniformity of said gap are of vital importance for the in-plane switching mode.
Further, according to the known TN mode, there is no dependence of the threshold voltage on the gap between the substrates (due to voltage responsitivity), while according to the in-plane switching mode, since the gap between the substrates independently contributes to the threshold voltage (due to field responsitivity) together with the gap between electrodes (Oh-e, et al. Appl. Phys. Lett. 67 (26), 1996, pp 3895-3897), particularly severe control of the gap between the substrates is necessary.